But I don't know how to successfully backstab
Only once have I been successful in hiding in shadows after the enemy has had sight of the character. All other times, hide in shadows has failed.
My hide in shadows/move silently scores are about 120% each. Do they have to go higher to succeed in hiding in shadows while battle has been joined? [I've seen it work with some of the thieves when escaping Irenicus' dungeon, and in the basement of the tanner/flayer quest]